1. Field of the Invention
The present invention relates to a thin foldable telephone or similar portable telephone and, more particularly, to a miniature portable telephone using a dynamic speaker and having, at the rear of the speaker, a back cabinet whose volume is variable to improve the acoustic characteristic of the speaker.
2. Description of the Related Art
Various kinds of portable telephones extensively used today include a thin foldable telephone and a cordless telephone. The prerequisite with such a portable telephone is that the acoustic characteristic of a speaker be improved in order to enhance the conversation quality, particularly the quality of received speech. This prerequisite has customarily been met by increasing the diameter of a speaker or by using a dynamic speaker. The problem with a dynamic speaker is that sound opposite in phase to sound issuing from the front of the speaker issues from the rear of the same. When the sound output from the rear of the speaker reaches the user's ear, it cancels the sound output from the front and often prevents the sound pressure from being high enough to be heard. In light of this, it is a common practice to form a closed space called a back cabinet behind the speaker.
The back cabinet, however, increases the overall size of the telephone. While the increase in size is not critical with a desk-top telephone or similar telephone made up of a body and a handset, it is vital when it comes to the portable telephone, cordless telephone and so forth which must be miniature.
Usually, when the portable telephone or cordless telephone is miniaturized, the quality of received speech is deteriorated. Particularly, the miniaturization aggravates the acoustic coupling between a speaker and a microphone mounted on the telephone. As a result, echoes are increased to deteriorate the quality of received speech and therefore the conversation quality of the telephone.